Direct numerical simulations of pool boiling curves including heater's thermal responses and the effect of vapor phase's thermal conductivity

Abstract

Effects of heater's thermal properties and vapor phase's thermal conductivity on saturated pool boiling above a large horizontal heater are simulated numerically based on an improved pseudo-potential liquid-vapor phase change lattice Boltzmann model. A transient conjugate heat transfer problem is under consideration, where the conjugate thermal boundary condition is imposed and heater's thermal responses during boiling processes are investigated. Saturated pool boiling curves from onset of nucleate boiling to critical heat flux (CHF), to transition boiling regime to stable film boiling regime are obtained numerically. It is found that the simulated critical heat flux (CHF) agrees reasonably well with existing analytical models. Also, the simulated boiling heat fluxes in stable film boiling regime are shown to be in good agreement with the existing analytical solution. Thus, this improved pseudo-potential liquid-vapor phase change lattice Boltzmann model is quantitatively validated. Simulation results demonstrate that there is significant maldistribution in temperature distribution near the top of heater surface in nucleate boiling regime, CHF point and transition boiling regime. As a result, two-dimensional heat conduction can not be ignored when evaluating heat flux closely beneath the heater's top surface. It is also shown that both heater's thermal conductivity and thermal mass (the product of density and specific heat at constant pressure) have no effect on CHF value as well as the boiling curve in nucleate boiling regime and film boiling regime for a thick heater. However, the transition boiling regime of the boiling curve moves to the left with the increasing heater thermal conductivity and heater thermal mass for a thick heater. Increasing the vapor theraml conductivity has no effect on CHF but would increase boiling heat flux in film boiling regime, and hence shortening the transition boiling regime.